Pertussis is an acute respiratory infection characterized by paroxysmal cough and inspiratory whoop for over 2 weeks. The incidence of pertussis has decreased markedly after the introduction of DTwP/DTaP vaccine ('DTwP/DTaP' hereinafter), but pertussis is still one of important infectious diseases as around 300,000 people are estimated to die of the disease each year throughout the world1) (Fig. 1).

The outbreak of pertussis has decreased significantly after immunization, but according to reports in the U.S., the incidence of pertussis has increased steadily among young infant and among adolescents and adults since the 1980s2-4). This phenomenon is possibly due to the waning immunity since childhood immunization and lack of natural booster events5-8). The immunity against pertussis is maintained only for around 5-6 years after immunization9). The increase of pertussis among adolescents and adults act as a source of pertussis for young infant before immunization with DTaP. Thus, it is important for adolescents and adults to maintain immunity to pertussis in order to decrease the incidence of pertussis in this age group and to protect young infant from pertussis, and Tdap vaccine ('Tdap' hereinafter) was introduced for this purpose.

In the U.S., two products of Tdap were approved in 2005, which are ADACEL™ (Sanofi-Pasteur, Totonto, Ontario, Canada) for those aged between 11-64 and BOOSTRIX®(GlaxoSmithKline Biologicals, Rixensart, Belgium) for those aged between 10-64. In Korea, ADACEL™ was approved in June 2009 and BOOSTRIX® in June 2010 by the KOREA FOOD & DRUG ADMINISTRATION for the prevention of diphtheria, tetanus and pertussis in adolescents and adults aged between 11-64.

The purpose of this statement is to provide the rationale and recommendation for adolescent use of Tdap vaccine in Korea (Box 1).

Pertussis is caused by Bordetella pertussis, a fastidious Gram-negative coccobacillus. It is one of the most highly communicable diseases, attack rates in susceptible household contact range from 70 to 100 percent. The main infection reservoir is adolescents or adults who show a light symptom or do not show any symptom, and the symptom is most severe when the disease occurs in infants younger than 6 months who have not completed the immunization. Pertussis-like syndrome may be caused by adenovirus type 1, 2, 3 and 5, Bordetella parapertussis, Chlamydia trachomatis, etc. Disease caused by Bordetella spp. other than B. pertussis is not prevented by pertussis immunization10).

Human is the only host of pertussis. Infection occurs by a direct contact or by respiratory droplets from a coughing patient. The outbreaks of pertussis have decreased markedly with the increase of immunization rate throughout the world (Fig. 1), but are reported intermittently. Despite the high immunization rate since the 1980s, the incidence of pertussis increased steadily in young infants as well as in adolescents and adults3, 4, 11). In particular, adolescents act as a reservoir and spread the disease to young infants with a high risk of complications12, 13). The recent increasing number of reports on the occurrence of pertussis is partially the result of active diagnostic examination and the advance of diagnostic technology. However, the increase of incidence in specific age groups suggests that pertussis may be prevalent continuously in the future if booster vaccination is not performed. Although data are not sufficient on the incidence of pertussis in Korea, according to recent statistics from the Korea Center for Disease Control and Prevention (KCDC), the report of pertussis shows an increasing pattern, so adequate actions should be taken. In Korea, large-scale outbreaks of pertussis have disappeared with the DTwP/DTaP immunization, but small-scale ones have been being repeated since 1995 and around 11.5 cases are being reported each year on the average. However, the accurate incidence is unknown due to the absence of pertussis diagnosis in primary hospitals and sentinel surveillance system, but according to disease surveillance reports by the KCDC, the number of cases increased in 2009 (Fig. 2). In addition, according to domestic data, pertussis was the cause of chronic cough in 2.9% of adults with chronic cough14).

The reasons for the steady rise in the incidence of pertussis include 1) a genetic change in B.pertussis15, 16), 2) the waning immunity, 3) poor management of adolescent and adult pertussis patients. In domestic studies as well, it was found from strains isolated from Korean pertussis patients that pertussis antigens are changing continuously over time17).

In general, pertussis has an incubation period of 7-10 days. The illness has three stages (catarrhal stage, paroxysmal stage, convalescent stage), and infectivity is highest in the catarrhal stage and falls rapidly afterwards, but transmission is possible for around 3 weeks. A characteristic paroxysmal cough occurs mainly at night and 15 fits around 24 hours on the average, maintaining similar intensity for 2-3 weeks and even up to 10 weeks. The frequency of complications and hospitalization is higher in infants and toddlers younger than 12 months than in older ones13, 18, 19). Among complications, the most common and fatal one is pneumonia caused by secondary bacterial infection. Besides, hypoxia-induced convulsion, encephalopathy, etc. may happen in infants, and paroxysmal cough may cause pneumothorax, nasal bleeding, subdural hematoma, hernia, rectal prolapse, etc.

Pertussis in adolescence shows considerable variation from mild to severe. When classic symptoms do not appear, the disease may not be distinguishable from other respiratory infections. Around half of adolescents diagnosed with pertussis cough for over 10 weeks, 72-100% show symptoms such as paroxysmal cough, dyspnea and sleep disturbance, 50-70% are accompanied with vomiting, 1-2% are hospitalized for pneumonia, etc., and 0.2-1% show symptoms such as convulsion and unconsciousness20, 21). It was found that pertussis was the cause of the cough in 13-20% of adolescents and adults with non-specific cough continuing for over a week22).

Pertussis is a highly communicable disease, and if one has contacted a pertussis patient and shows a characteristic pattern of cough he/she may be suspected to have pertussis. Bacterial identification through culture study is the most accurate diagnosis method but the culture is quite a complicated process. Recently, the test based on polymerase chain reaction (PCR) is being used increasingly and it is known to show a positive rate 2-3 times higher than that of culture study in typical cases23-25). In a serologic test, the increase of pertussis antibody should be proved in serial serum at the acute stage and the convalescent stage.

Along with the use of Tdap, related research should be made continuously. It is necessary to monitor the effect and safety of vaccination, and additional studies should be conducted on change in the epidemiology of pertussis in response to vaccination in Korea.